Mg face hurt while I was walking down the street just now.

hurt是过去分词,Mg face hurt 是我的脸受伤了,这是被动语态,所以用过去分词形式,后面的while只是引导 I was walking down the street just now这个从句

while是引导从句I was walking down the street just now的。 这里的HURT是过去史,因为后面是JUST NOW,(刚刚的意思).